1. Field of the Invention
This invention relates to capacitive touch detection such as in a system having capacitive touch buttons or knobs which respond to finger contact.
2. Description of the Prior Art
Various types of touch contact systems utilise the fact that the human body has a degree of capacitance to ground. Typically, since there will be some stray capacitance in the system even when the button is not being touched, touch detection is generally achieved by virtue of a detected capacitance being greater than a certain threshold, this threshold being sufficiently higher than the expected stray capacitance. A problem arises in systems where it is possible or even necessary for more than one button to be touched simultaneously. This has the effect of "sharing" the capacitance of the human body amongst the detection circuits associated with the buttons being touched. Accordingly, it is possible for the reduced capacitance (by virtue of the sharing effect) being below the threshold so that there is no touch detection. The effectiveness of such a system is thus dependent on the number of buttons being touched.
One example of a system in which the problem may arise involves signal mixing consoles such as audio mixing consoles which may be provided with motorized faders (potentiometers controlling the volume or other signal level). The motorized faders can automatically be set to predetermined levels by virtue of the motor drive. If the operator should then wish to adjust a fader manually, it is necessary to provide some means of detecting when this is being done, thereby preventing the motor acting against the operator when such detection has taken place. A capacitive detection technique has been used for this purpose. The faders have electrically conducting control knobs which are electrically charged by the detection circuit. When a finger is placed on a control knob, the change in capacitance of the control knob is detected by the detection circuit and the fader motor is inhibited. Even although it is known to provide a separate capacitance detector for each control knob, the above capacitance-sharing effect can still lead to lack of detection if, as is common in audio mixing applications, the operator is touching more than one fader at the same time. It can be possible to design detection circuits which operate reasonably reliably if two faders are touched simultaneously, but when three or more faders are touched, the circuits become unreliable. If, in order to overcome this difficulty, the sensitivity of the detection circuits is increased (i.e. the capacitance threshold is lowered), this tends to cause spurious inhibiting of the fader motors since the stray capacitance of the circuit will be much closer to the threshold and only a slight increase will trigger the motor inhibiting circuit.
Another problem with many existing touch detection circuits is their relative complexity. Such circuits are sometimes designed to respond to signal phase changes caused by the increased capacitance resulting from touch contact, and these circuits can be complex and hence expensive, which is a significant disadvantage if the particular application requires a large number of such detection circuits.
It has been proposed for audio mixing consoles to have a relatively small number of faders and control knobs (for controlling gain, equalization, audio processing and the like) in order to control a much larger number of audio channels; for example, one set of controls can be used to control up to eight different channels. This is achieved by the use of assignable controls, so that a control knob or fader can be temporarily assigned to a particular channel to make an adjustment, and subsequently used to make other adjustments to other channels, previous adjustment values being stored by the equipment and applied to the corresponding channels. In particular, a single set of equalization and processing controls may be shared between a number of channel faders. Whereas this saves on the number of control knobs required on the mixing console, and thus reduces the size and complexity of the console (which is a particular advantage in consoles having a large number of channels to be mixed), it can lead to operator confusion as to which channel is being controlled at any particular time, and also the operator may be adjusting the gain of one channel while the equalization controls are assigned to another channel.